There are two types of electroprocessing system for electrically processing metals, namely an electrolytic processing apparatus and an electric discharge processing machine. The first-mentioned apparatus (the electrolytic processing apparatus) employs an electrolyte such as sodium nitrate and sodium chloride, which fills the gap between a work and an electrode. In operation, the electrolyte is circulated at a high speed and D.C. current is made to flow from the work to the electrode while a suitable measure is taken for removing electrolytic products which impair the stability of the electrolysis, for example eluated intermetallic compounds, metal ions and hydrogen gas. This type of electrolytic processing apparatus is shown, for example, in Japanese patent laid-open No. 71921/1986 and Japanese patent laid-open No. 44228/1985.
The second-mentioned type of apparatus (the electric discharge processing apparatus), employs a bath of a processing liquid such as water, kerosene and the like in which a work is disposed to oppose an electrode with a small gap therebetween. In operation, the work and the electrode are connected to each other so as to cause a momentary sparking discharge or a transient arc between the work and the electrode, thereby processing the work by the energy of the discharge. This type of processing apparatus is shown, for example, in Japanese patent publication No. 26646/1985 and Japanese patent laid-open No. 177819/1985.
An example of this power supply system is shown in FIG. 9. This power supply system has a power source unit 178 connected to a capacitor 176 which is adapted to cause an electric discharge between an electrode 174 and a work 172. The capacitor 176 is connected at its one end to the work 172 through a discharge switch 180 and at its other end to the electrode 174 through grounding. The power supply unit 178 has a transformer 182 and a rectifier 184. A.C. power is transformed to a lower voltage by the transformer 182 and is rectified by the rectifier 184, and the resulting D.C. power is used to charge up the capacitor 176. As the discharge switch 180 is closed, an electric discharge is caused between the work 172 and the electrode 174 so as to machine or process the work 172.
Power supply systems are also known in which, as disclosed in Japanese patent laid-open No. 3532/1985, an auxiliary capacitor is provided so as to enable the charging voltage to be varied, as well as an apparatus in which, as disclosed in Japanese patent laid-open No. 59097/1985, a second capacitor is provided to control the charging voltage at a constant level. Japanese patent laid-open No. 38819/1986 discloses a plurality of capacitors of different capacitances for selective use so as to provide different discharge voltages.
All of these known power supply systems suffer from a common problem in that, since the processing is conducted by electric discharge from a single capacitor at a time, a large noise pulse is generated in the power supply side when a large discharge current is produced, thus adversely affecting the surrounding environment.
In order to control the quantity of charges discharged between the work and the electrode, it is necessary to impart a voltage drop characteristic to the discharge switch or to connect a resistance in series to the discharge switch. In consequence, however, power is consumed wastefully.
The state of the surface of the work processed by an electrolytic processing apparatus depends on various factors such as current density, work gap and density of the processing liquid. When one of these factors is insufficient, for example when the current density has come down below a predetermined level, the processing precision is impaired, so that the desired degree of luster is not achieved. Hitherto, various measures have been taken for the purpose of ensuring a sufficiently large margin of the processing conditions so as to obtain a quantity of the electrostatic charge large enough to conduct the processing. For instance, it has been proposed to set the charging voltage at a level higher than a predetermined level. It has also been proposed to reduce the length of time over which the discharge is conducted A setting of the charging voltage at a high level, however, causes the electric current density to increase to an impractically high level at the peak of the discharge current. On the other hand, the reduced time length of the discharge increases the number of discharging cycles needed for a given amount of processing, with the result that the processing time is increased undesirably.
The known electrolytic processing apparatus in general suffers from the following vital defect. Namely, it is impossible to obtain a uniform flow velocity of the electrolyte through the gap between the electrode and the work when the work has a complicated shape, such as a three-dimensional recess with a bottom. In addition, different levels of concentration of electrolytic products are developed between the inlet and outlet side, even when a large pressure of the electrolyte is applied to the discharge gap. This means that different portions of the discharge gap produce different processing conditions, even if the discharge current is developed uniformly over the whole area of the gap. In consequence, it becomes difficult to precisely transfer the electrode to the work and, hence, difficult to obtain a high precision of the processed work surface.
On the other hand, the known electric discharge processing systems in general exhibit a comparatively high level of efficiency in the range of fineness of the work surface in terms of roughness (Rmax) of up to 20 .mu.m. For attaining a higher degree of surface fineness, it is necessary to employ a very small processing current of less than 1 A. In consequence, the processing time is impractically long, particularly when the work has a large surface to be processed. When the area of the processed surface is large, the electrostatic capacitance between the work surface and the electrode is increased, so that it becomes difficult to delicately control the discharge current, with the result that the desired level of surface fineness is not achieved.
In ordinary discharge processing using an insulating oil, the processed surface usually has a hardened surface layer which is minutely cracked to a large depth. On the other hand, wire discharge processing making use of pure water generates a softened layer. Both types of electric discharge processing, therefore, cannot provide the desired quality of the surface. When the product surface is required to have a high degree of precision or extended life, lapping and other surface polishing operation is conducted. Thus, much time and labor are required for finishing the surface after the electric discharge processing.
Accordingly, an object of the present invention is to provide a power supply system for an electrolytic processing apparatus which is capable of suitably controlling and maintaining electric charges discharged between an electrode and a work which oppose each other across a processing liquid, while reducing noise generated in the power supply side so as to diminish any unfavorable effect which may otherwise be caused on the surrounding environment.
Another object of the present invention is to provide a power supply system of an electrolytic processing apparatus which is capable of controlling the amount of charges discharged between a work and an electrode so as to finish a complicated work surface (such as a three-dimensional shape) in a short time and with a high degree of precision.